Electrometallurgical processes are generally carried out in undivided electrochemical cell containing an electrolytic bath and a multiplicity of anodes and cathodes; in such processes, such as the electrodeposition of copper, the electrochemical reaction taking place at the cathode, which is usually made of stainless steel, leads to the deposition of copper metal on the cathode surface. Normally cathodes and anodes are vertically arranged, interleaved in a face-to-face position. The anodes are fixed to suitable anodic hanger bars, which in their turn are in electrical contact with positive bus-bars integral with the cell body; the cathodes are similarly supported by cathodic hanger bars which are in contact with the negative bus-bars. The cathodes extracted at regular intervals, usually of a few days, to effect the harvesting of the deposited metal. The metallic deposit is expected to grow with a regular thickness over the entire surface of the cathodes, building up with the passage of electric current, but it is known that some metals, such as copper, are subject to occasional formation of dendritic deposits that grow locally at increasingly higher rate as that their tip approaches the surface of the facing anode; inasmuch as the local distance between anode and cathode decreases, an increasing fraction of current tends to concentrate at the point of dendrite growth, until the onset of a short-circuit condition between cathode and anode occurs. This obviously entails a loss of faradic efficiency of the process because part of the supplied current is dispersed as short-circuit current rather than being used to produce more metal. In addition, the establishment of a short-circuit condition brings about a local temperature rise in correspondence of the contact point, which in turn is the cause of damage to the anode surface. With the anodes of the older generation, made out of lead sheets, the damage is generally limited to the melting of a small area around the dendrite tip; the situation is however much more serious when present-day anodes made of catalyst-coated titanium foraminous structures such as meshes or expanded sheets are used. In this case, the lower mass and thermal capacity of the anode, coupled with the higher melting point, often involves widespread damages, with a substantial anodic area that gets entirely destroyed. Even when this doesn't occur, there's the risk that the tip of the dendrite, opening its way across the anode meshes, may get welded thereto, making the subsequent extraction of the cathodes problematic at the time of product harvesting.
In a more advanced generation of anodes, the catalyst-coated titanium mesh is inserted inside an envelope consisting of a permeable separator—for instance a porous sheet of polymeric material or a cation-exchange membrane—fixed to a frame and surmounted by a demister, as described in concurrent patent application WO2013060786. In this case, the growth of dendritic formations towards the anodic surface entails the further risk of piercing of the permeable separator even before they reach the anodic surface, resulting in the inevitable destruction of the device.
It has thus been evidenced the need to provide a technical solution allowing to prevent the harmful consequences resulting from the uncontrolled growth of dendritic deposits on the cathodic surfaces of metal electrowinning cells.